Antenna incorporating active elements



July 23, 1963 A. F. WICKERSHAM, JR., ETAL 3 ANTENNA INCORPORATING ACTIVEELEMENTS Filed May 27, 1960 DIPOLE ANTENNA} 2 Sheets-Sheet 1 llu Ilb |2b:20 20 2| TEXNS'NQEON LINE 4 7 DIODE x DIPOLE ELEMENT IN V EN TORSARTHUR F. WICKERSHAM JR, IE .Zl

ATTORNEY y 1963 A. F. WICKERSHAM, JR., ETAL 3,098,973

ANTENNA INCORPORATING ACTIVE ELEMENTS 2 Sheets-Sheet 2 Filed May 27,1960 O O o O POTENTIAL ACROSS DIODE MILLIVOLTS) IN V EN TORS ARTHUR F.WICK ERSHAM,JR.

R088 L. BELL BYru%M/M ATTOBNEY Ila-4 United States Patent Ofi ice3,098,973 Patented July 23, 1963 3,098,973 ANTENNA INCORPORATING ACTIVEELEMENTS Arthur F. Wickersham, In, Sunnyvale, and Ross L. Bell, SanJose, Calif., assignors to Sylvania Electric Products Inc., acorporation of Delaware Filed May 27, 1960, Ser. No. 32,235 2 Claims.(Cl. 325375) This invention relates to antennas, and more particularlyto an active antenna in which antenna and electronic amplificationfunctions are fully integrated within one structure.

In present-day radio practice, it is common to regard an antenna as aseparate unit connected by means of a transmisison line to anotherseparate unit which may be a receiver or transmitter. In the case of areceiving station comprising an antenna and receiver combination, it isthe purpose of the antenna to intercept radio energy in space and todeliver such energy to a receiver. It is the purpose of the receiver toamplify and modify the signal and so achieve a desired signalutilization. While the discussion in the specification which followsrelates to the antenna-receiver combination, it will be understood thatthe underlying principle of the invention applies, with appropriatemodification, to an antenna-transmitter combination.

Briefly, the present invention, in its preferred form, comprises acombination of a tunnel diode and a simple dipole antenna. The tunneldiode is connected across the center of a series-fed type dipole, thecircuit constants of which are utilized as constants for a tunnel diodeamplifier, the latter being suitably biased into its negative resistanceoperating region. This antenna is operated with a matched transmissionline conventionally connected to the dipole, the received signal beingtransferred in phase to the transmission line with gain provided by thetunnel diode amplifier. Alternatively, if no transmission line isutilized, the received signal is amplified and re-radiated by thestructure.

An object of the present invention therefore is to combine the functionof the antenna with at least one of the functions, the amplifyingfunction, of the receiver and to provide a single physical embodimentcapable of performing both functions simultaneously.

A further object is the provision of an antenna assembly whicheliminates the need for a receiver structure spatially separated fromand independent of the antenna. By eliminating the separate receiver,the invention also eliminates the need for radio frequency transmissionline between the antenna and receiver. This not only removes powerlosses associated with the transmission line, but also achieves furthereconomy and simplicity in the overall system by doing away with RFrotary joints, bends, twists, etc.

Another object is the provision of an antenna-receiver combination thatavoids problems associated with impedance matching of the antenna to thetransmission line and matching of the transmission line to receiver.

A further object of the invention is to reduce electric-a1 noise,eliminate components, and improve efiiciency of operation, reduce theoverall size of the antenna-receiver package, and to greatly reduce thecost of fabrication of such systems.

Another object is to provide a simple unit which may be used either as areceiving station (antenna-receiver combination embodied in asingle-physical unit) or as an active scatterer or repeater, i.e., asimple unit capable of reradiating energy of amplitude greater than thatreceived. Such a unit, combining functions usually ascribed to antennaand receiver and transmitter, and used either as a receiving station oran amplifying repeater station, is not a passive component. Therefore,it is called an active antenna in this specification.

A further object of the invention is to provide an active antenna ofsimple and rudimentary form which may be regarded as a sub-component ofmore elaborate antenna arrays and configurations. The use of theelementary sub-unit to form elaborate arrays may be either for thepurpose of obtaining results now achievable only with a complex ofantennas and receivers, or for obtaining results not realizable withpresent combinations of separate antennas and receivers.

These and other objects of our invention will become apparent from thefollowing description of a preferred embodiment thereof, reference beinghad to the accompanying drawings in which:

FIGURE 1 -is a side elevational view of dipole antenna embodying ourinvention;

FIGURE 2 is a greatly enlarged view of the center part of FIGURE 1, aportion of the antenna being shown in section to illustrate details ofconstruction;

FIGURE 3 is a diagramof an equivalent circuit of our active antenna;

FIGURE 4 is a curve showing the current-voltage chanacteristics of atunnel diode used in our active antenna;

FIGURE 5 is an equivalent circuit of a tunnel diode; and

FIGURE 6 is an equivalent circuit of a tunnel diode and the dipoleantenna.

Referring now to the drawings, a preferred embodiment of the inventionis shown in FIGURE 1 as a cylindrical dipole 10 having elements 11 and12 center-fed by connection to a twin-wire transmission line 13, 14- oralternatively by a coaxial line through a balun (not shown). The mainbody portions and 12a of the dipole elements comprise hollow cylindricalconductors which have tapped inner ends, the inner end of element 1-1being indicated at 15 in FIGURE 2, by means of which frusto-conicallyshaped inner caps 11b and 1211 are removably secured to the main bodiesof the elements.

Mounted within one of the caps, for example, cap 11b, is an electriccell and biasing network which biases a tunnel diode indicated at :18preferably mounted in and supported by cap 11b. The associatedelectrical network is indicated schematically at 19 and is provided forthe purpose of biasing tunnel diode 18 for operation in its region ofnegative resistance as will be explained below. Leads 20 and 21 connectthe tunnel diode electrically to dipole element 11 (through cap 11b) andto element 12 (through cap 12b), respectively. Biasing network 19 has alead 22 which extends through an insulator 23 in cap 11b for connectionat 24 to cap 12b of the opposite dipole element. The purpose of leads 20and 22 is to make electrical connection of the diode and the biasnetwork represented by diode 18 and circuitry 19, respectively, to theamplifier network represented by and contained in the physical dipoleelements. The ar-ms of the dipole are not only important antennaelements, but are also physically and electrically important tankcircuit elements of the tuned amplifier circuit which is described morefully below.

A schematic diagram of the biasing network for tunnel diode .18 is shownin FIGURE 3. The source of bias potential may be and preferably is asmall cell or battery 25, such as a common flashlight cell or a mercurycell. Resistor 26 drops the bias potential to a value near its desiredlevel and potentiometer 27 provides a means of fine adjustment of thebias potential. A suitable radio frequency choke 28 prevents highfrequency energy from being coupled through the direct current supplycircuit and shunt resistor 30 across the diode and choke provides 3 alow impedance bias supply necessary for stability of operation. Dipoleelements 11 and 12 and tunnel diode 18 are electrically connected inseries in this circuit so that when the proper bias potential is appliedto diode 18, these elements may be utilized as an amplifier.

The bias supply elements, being few in number and small in size, occupya minimum of space so that they may readily be mounted within the cap ofa hollow dipole element having a diameter of one inch. Access to thebias supply for adjustment of potentiometer 27, replacement of battery25, or removal and inspection of diode 18 is readily accomplished byremoval of cap 11b from the main body 11a of the dipole.

Dipole elements 11 and 12 are shown in FIGURE 3 to indicate theirapproximate location in the circuit. The function of the dipoleelements, as far as circuit operation is concerned, is not only tointercept or receive an electromagnetic wave, thereby producing a signalvoltage, but also to supply inductive reactance with some distributedshunt capacitance so that these elements together with the tunnel diode18 may properly amplify radio frequency signals.

In order to better understand the operation of tunnel diode 18 as acomponent in a radio frequency amplifier circuit, a plot of thecurrent-voltage characteristic of a commercially available tunnel diodeis shown in FIG- URE 4. The vertical scale indicates current through thediode and the horizontal scale shows a potential impressed across thediode. It is seen that the characteristic curve 31 displays a negativeresistance between 30 and 250 millivolts. It is the purpose of theelectrical network shown in FIGURE 3 to bias the diode so that it isoperating in this region of negative resistance. The dotted line 32passing through point A of the curve is the slope of the characteristiccurve in the neighborhood of point A and represents a negativeconductance or load line. For stable operation, this diode must have adynamic load line slope equal to or greater than that of the static loadline. Stable devices characterized by slopes of magnitude greater thanthe slope of the static load line are termed short-circuit stable. Adynamic load line for stable operation is shown at 33 in FIGURE 4. Alsoshown in this figure is a load line 34 with less slope than the staticload line; and, since this load line of less slope crosses thecurrent-voltage curve at several points (points A, B and C), it depictsa condition of unstable or oscillatory operation. It is noted that theload line is the threshold or dividing line between stable andoscillatory conditions.

A radio frequency circuit which is the equivalent of a. tunnel diode isshown in FIGURE wherein the resistance 35 is a series resistance,resistor 36 is a negative resistance and capacitance 37 is the shuntcapacitance of the diode. The resistance 36 will be negative as long asthe diode is biased to operate on the negative slope of thecurrent-voltage characteristic as shown in FIGURE 4. Resistance 35 isthe dissipative resistance of the diode including losses inherent in theconnections to outside circuits, and, in general, is small in valuecompared to resistance 36. While capacitance 37 is relatively large,i.e., for an abrupt junction with 4 l0 carriers/cm. in the bulkmaterial, the capacitance will be approximately 5 ,uf./cm. of thejunction area.

FIGURE 6 illustrates the equivalent radio frequency circuit of the diodeas shown in FIGURE 5 combined with the equivalent of the dipole antennarepresented by the broken line rectangle 40. The latter includes aninduct ance 41 corresponding to the distributed inductance of the twoarms of the dipole, and a capacitance 42 representing the distributedshunt capacitance of the dipole arms. By making the length I, see FIGURE1, of each dipole element slightly longer than the resonant length,i.e., slightly greater than a quarter wavelength at the midpoint of theoperating frequency range, the result is that the equivalent reactanceof the dipole is inductive as represented by the equivalent inductance43 in FIG- URE 6. Resistance 38 represents the sum of resistance 35 inFIGURE 5 and the radiation resistance of the dipole. Voltage generator44 represents the signal input to the antenna from interceptedelectromagnetic waves when the antenna is used for receiving purposes,and corresponds to a transmitter when the antenna is used fortransmission purposes or both if the antenna is used as a repeaterstation. The voltage E developed by source 44 is amplified by thecircuit and appears as a larger voltage E as indicated at the right sideof FIGURE 6. The signal E is carried by transmission lines 13, 14- toassociated utilization circuits or may be re-radiated by the antennaitself if such action is desired.

It should be noted that in an active antenna constructed in accordancewith our invention certain individual components provide both antennaand amplifier functions. For example, the arms 11 and 12 of the dipoleare not only important antenna elements, but are also physically andelectrically important tank circuit elements in the amplifier circuit.These dual purpose elements perform the antenna and receiver functionssimultaneously, resulting in a compact, eflicient and economical device.

The equivalent circuit illustrated in FIGURE 6 may be used to show thata dipole length which is slightly greater than the normal resonantlength provides conditions under which the combined circuit acts as astable amplifier. T0 find the required increase in length, the necessaryinductance, L, must be determined from the circuit equations:

is impressed radio frequency voltage.

Elimination of E from Equations 1 and 2 leads to:

The frequency ar now is further restricted by choice of the new lengthof the dipole to correspond to a frequency at which the above circuitadmittance is real, but that frequency is still represented by thesymbol o Under this condition the imaginary part of the admittanceequation can be solved for 01 For given values of C, and G, L can bedetermined; and from recorded values of L, a corresponding dipole lengthcan be found. Thus a resonant frequency, w of the circuit can beassociated with a given length of dipole, where the dipole is operatingslightly above its own natural resonant frequency.

The stability of the current when it is operating in the conditiondescribed above is now examined. Recalling that the tunnel diode is ashort-circuit stable device, stable operation is obtained when themagnitude of the admittance in shunt with the diode is larger than theabsolute value of the negative conductance:

For a first approximation, assume that the frequency 0: is not farremoved from the natural undamped resonant frequency of the circuit andso (0 can be replaced by l/LC in the last term of the numerator (butreplaced by Equation 4 elsewhere). This results in ZI I It is thus seenthat stability is achieved and it remains to show that amplification canbe obtained under the same operating conditions.

Examination of the conditions for amplification dictates that anexpression for the power delivered to the load, represented by radiationresistance R, must be found. Eliminating (.0 by using Equation 4 and theexpression for admittance, Equation 3, an expression for the current inR is obtained:

CE RC LG consequently, the power dissipated in the load will be =E Km)(9) When the denominator of the power expression vanishes, i.e., whenthe power becomes indefinitely large and this condition approximatelycorresponds to the condition for maximum gain,

If again,

is equal to 50 ohms and R=50 ohms the condition for maximum gain is nowsatisfied by now taking G=0.02 ohms. For these values of the parametersthe stability condition reduces to IZRG, which is satisfied with theequality sign. Thus, it is seen that maximum gain is achieved at theedge of stability; if less than maximum gain is acceptable, fully stableoperation is achieved.

From the foregoing description it will be seen we have provided aself-contained antenna-amplifier which is capable of functioning as atransmitting element or a receiving element or as a repeater element.The dipole elements '11 and 12, in the preferred embodiment hereindescribed, in essence simultaneously provide inductive reactance as wellas radiation resistance, the radiation resistance being essential to theantenna function and the reactance being essential to the amplifyingfunction. Therefore, both of these functions are provided by a singlesimple structural element.

While the above described preferred embodiment consists of a simpledipole as a radiating element, it will be apparent to those skilled inthe art that the invention may be incorporated in a folded dipole, aunipole, a loaded unipole, or in various other types of elementaryradiators. Accordingly, the invention is not to be limited to thepreferred embodiment described above but the scope thereof is defined inthe appended claims.

We claim:

1. A dipole antenna comprising a pair of axially aligned hollowconducting elements having their inner adjacent ends axially spacedapart, a tunnel diode mounted Within one of said elements and having twoleads, bias voltage supply means mounted within one of said elements,means for electrically connecting the leads of said diode to the innerends respectively of said elements, means for electrically connectingthe output of said supply means across said elements whereby the supplymeans and the elements and the diode are connected together inelectrical series, the output of said supply means being of suchmagnitude that said diode operates in the negative resistance region ofits voltage-current characteristic, each of said dipole elements havinga length slightly greater than its resonant length at the midpoint ofthe operating frequency range of the antenna whereby said elements andsaid tunnel diode jointly function as a microwave amplifier of signalsimpressed on said elements.

2. The antenna according to claim 1 in which the inner end of at leastone of said elements is removably secured to the remainder of theelement.

References Cited in the file of this patent UNITED STATES PATENTS1,930,505 Brown Oct. 17, 1933 2,169,358 Hollmann Aug. 15, 1939 FOREIGNPATENTS 158,879 Australia Sept. 16, 1954 OTHER REFERENCES Article bySommers, Jr., in Free. I.R.E., July 1959, pages 1201-1206.

1. A DIPOLE ANTENNA COMPRISING A PAIR OF AXIALLY ALIGNED HOLLOWCONDUCTING ELEMENTS HAVING THEIR INNER ADJACENT ENDS AXIALLY SPACEDAPART, A TUNNEL DIODE MOUNTED WITHIN ONE OF SAID ELEMENTS AND HAVING TWOLEADS, BIAS VOLTAGE SUPPLY MEANS MOUNTED WITHIN ONE OF SAID ELEMENTS,MEANS FOR ELECTRICALLY CONNECTING THE LEADS OF SAID DIODE TO THE INNERENDS RESPECTIVELY OF SAID ELEMENTS, MENS FOR ELECTRICALLY CONNECTING THEOUTPUT OF SAID SUPPLY MEANS ACROSS SAID ELEMENTS WHEREBY THE SUPPLYMEANS AND THE ELEMENTS AND THE DIODE ARE CONNECTED TOGETHER INELECTRICAL SERIES, THE OUTPUT OF SAID SUPPLY MEANS BEING OF SUCHMAGNITUDE THAT SAID DIODE OPERATES IN THE NEGATIVE RESISTANCE REGION OFITS VOLTAGE-CURRENT CHARACTERISTIC, EACH OF SAID DIPOLE ELEMENTS HAVINGA LENGTH SLIGHTLY GREATER THAN ITS RESONANT LENGTH AT THE MIDPOINT OFTHE OPERATING FREQUENCY RANGE OF THE ANTENNA WHEREBY SAID ELEMENTS ANDSAID TUNNEL DIODE JOINTLY JUNCTION AS A MICROWAVE AMPLIFIER OF SIGNALSIMPRESSED ON SAID ELEMENTS.